The present invention relates to a method and device for measuring fine particles in order to determine a physical quantity in proportion to sedimentation rate of the fine particles by measuring in time sequence the position of the fine particles to a cell at the time of imparting the displacement by pulse or vibration to both a suspended solution of the fine particles and the cell containing the above solution.
The objective in a conventional method for obtaining the hydrodynamic properties of particles suspended in a liquid medium is the measurement of the movement of the solvent-solution boundary either under a centrifugal force, in the case of sedimentation measurement, or under an electric force in the case of electrophoresis. Another approach also involves the measurement of the deformation of the boundary in the case of diffusion measurement. In any case, the observation must be carried out for a long time, several hours for instance, and any disturbances during this period are accumulated and produce undesirable effects in the final result.
Recently, the availability of coherent light by laser oscillation has made it possible to measure the relative displacement of particles with respect to the cell wall at frequent intervals by measuring the interference of the scattered light from particles and the cell wall. The interference of the scattered light gives rise to beatnotes in a square-law detector such as a photomultiplier, if these light beams have different frequencies. When light is scattered by a moving particle, it is frequency shifted by the Doppler effect. Therefore, the details of the relative movement can be obtained by analyzing the frequency spectrum of the output current from the photoelectric detector. This so-called optical beating device has already made possible rapid measurements of diffusion and electrophoresis.
This invention opens up a possibility for rapid measurement of sedimentation phenomena, especially by introducing a vibrational force field instead of a constant centrifugal force which requires a relatively large mechanical installation.